In general, an LNG carrier is provided with a plurality of cargo tanks 51 as shown in FIG. 4. The cargo tank 51 stores low-temperature (about −162° C.) LNG, loaded at a loading site, at atmospheric pressure and carries it to an unloading site of destination. On this voyage, the pressure in the cargo tank 51 is increased by boil-off gas (hereinafter referred to as BOG) generated from the stored LNG.
Therefore, on a conventional LNG carrier, in order to control the cargo tank pressure so as to be within a specified range to avoid the increase in pressure in the cargo tank 51, the cargo tank 51 is connected to a burning system (a complete system for using BOG as boiler fuel) 52 via regulating valves 53, a compressor 54, and the like, so that BOG evaporating from the cargo tank 51 is compressed by the compressor 54 and then is supplied to the burning system 52, where the BOG is burned. The LNG carrier is provided with a standby compressor 55 having the same construction as that of the compressor 54. This standby compressor 55 is disposed in parallel with the compressor 54 in case the compressor 54 should fail.
After LNG is unloaded at the unloading site, the LNG carrier makes her voyage in a ballast condition again to the loading site. At this time, LNG of about 2% of its quantity at the full load time is left in the cargo tank 51. The reason for this is that the tank is then prevented from being damaged by sudden loading of low-temperature LNG in a complete ballast condition.
On such a voyage in a ballast condition, since the temperature in the cargo tank 51 rises, the remaining low-temperature LNG is sprayed by a spray pump, not shown, via the regulating valve 53 to cool a plurality of locations at the bottom part and the upper part of the cargo tank 51 while the temperature state in the tank is observed.
It is more advantageous in terms of cost to return BOG generated in the cargo tank 51 into the cargo tank 51 and recover it for use as original LNG than to use the BOG as an auxiliary fuel for a boiler and the like.
In the above-described conventional method for controlling the pressure in the cargo tank 51, however, a system is used in which BOG generated in the cargo tank 51 passes through the compressor 54 (or the standby compressor 55) and is supplied to the burning system 52, where the BOG is burned. Therefore, the BOG is used merely as an auxiliary fuel for a boiler and the like, so that it is difficult to meet a demand for cost reduction.